This invention relates to a method of and apparatus for controlling a plasticizing process of a resin in an in-line screw-type injection molding machine for the purpose of obtaining a resin having entirely uniform temperature distribution after the resin has been plasticized by controlling back pressure of a screw.
Generally, in an injection molding machine having an in-line screw-type plasticizing mechanism, a resin fed into a heating cylinder of the mechanism is melted and plasticized therein by heating energy applied by a heater and shearing energy caused by the rotation of the screw. The heating energy is applied to the resin in proportion to the time during which the resin stays in the heating cylinder. However, it is rather rare that the entire resin measured and fed into the heating cylinder by one resin measuring step is used for one injection molding shot, and usually the mixture of the resin measured and fed by two or more measuring steps is injected by two or more molding shots, so that the temperature distribution of the resin to be injected by one molding shot is not uniform. In addition, since the depths between the respective screw threads are not equal throughout the entire length of the screw, heat transfer rate varies at respective positions of the screw and therefore, the temperature distribution of the resin becomes non-uniform. Moreover, the shearing energy also varies and is not constant for the reason that the effective length of the screw (i.e. the length from a hopper for supplying a resin to the front end of the screw) decreases as the screw is moved backwardly after the completion of the plasticizing process.
In order to make uniform the temperature distribution of the resin after the resin plasticization step, it is necessary to compensate for the variations in the heating energy and the shearing energy, for example, by controlling the number of revolutions and/or the back pressure of the screw. The control of the number of revolutions of the screw changes the shear rate (i.e. shearing energy) of the screw and this control affects the entire resin in molten state existing in the spaces between the threads of the screw. The control of the back pressure increases a pressure to be applied to the threads of the front end of the screw thereby increasing the mixing effect of the resin existing in the spaces between the threads of the screw along a limited length of the screw which is subjected to the effect of the back pressure.
According to a prior art method of controlling temperature distribution of a resin after the plasticization step, it is necessary to gradually or stepwisely increase or decrease the number of revolutions and the back pressure of the screw. However, according to this method, it takes much time to increase the back pressure of the screw, and moreover, it is obliged to decrease the speed of the backward movement of the screw during the increasing of the back pressure. Thus, the resin plasticizing process in the prior art takes much time for one injection molding cycle and the production efficiency of molded products is lowered.